The field of this invention is the synthesis of hydroquinone by acid cleavage (Hock-splitting) and recovery of hydroquinone from the reactor effluent.
The acid catalyzed cleavage or rearrangement of isopropylbenzene hydroperoxide was shown by Hock and Lang, Ber. 77B, p. 257 (1955), thus the name Hock-splitting. This reaction has been developed and used in the manufacture of phenolic compounds, including hydroquinone by the cleavage of p-diisopropylbenzene dihydroperoxide.
There has been a continuing effort on the part of those working in the field to develop a process which has a high level of product purity. The reason for this is that hydroquinone must be a white crystal or crystalline powder which meets ASA specification PH 4.126-1962 in order to be photographic grade. A high standard for whiteness is imperative for photographic grade material. Hydroquinone is used widely as the main ingredient in black and white film print developing.
Hydroquinone is also useful as a polymerization inhibitor and as an antioxidant. Hydroquinone itself and derivatives, such as 2,5-di-tert.-butylhydroquinone and butylated hydroxyanisole (BHA), are used for the prevention of oxidation in animal fat and aviation fuels.
By-products of the Hock-splitting (or rearrangement) reaction, referred to as tars, are believed to be the major impediment to high purity. Among the components of these tars are: p-isopropylphenol, .alpha.-hydroxy-p-isopropylphenol, p-diisopropylbenzene, p-isopropenylphenol, isopropenyl acetophenone, and dimers and trimers of the product and by-products. These impurities and by-products must be removed from the rearrangement product mixture in order to obtain a good color grade and high purity product.
Two processes applying the Hock-splitting reaction to p-diisopropylbenzene dihydroperoxide are found in the U.S. Pat. Nos. 3,884,983 and 3,968,171. These processes both employ benzene as a reaction solvent and as an extractant in removing tars.
Benzene has been the subject of regulatory action. The Occupational Safety and Health Administration has promulgated an emergency temporary standard bringing the average allowed exposure of a worker to benzene down to one part per million on a time-weighted basis, and allowing no more than five parts per million (ppm) peak exposure in factories during any 15-minute period. Even prior to this regulation the exposure was limited to 10 ppm. Thus, it is desirable to limit as much as possible the use of benzene in the process for making hydroquinone.
There are several processes, besides the two already mentioned, which use as the first five unit operations: rearrangement reaction, neutralization, removal of salts from the mixture, distillation and extraction to remove tars. In Japanese Pat. No. 69/017762, the extractant is an aromatic hydrocarbon such as benzene or toluene. U.S. Pat. No. 3,798,277 is a similar process except that its extractant is a halogenated hydrocarbon such as methylene chloride and alkyl ethers such as isopropylether, both of which are hazardous types of materials. Other extraction pg,4 purification processes are found in Japanese Pat. Nos. 74/18835 (CA 81:13265n) and 74/18836 (CA-81:13264m).
A significant advance in the processing technology for phenolic compounds occurred when the process for isolating mono and dihydroperoxides by selective extraction first into a caustic solution, then into a water-in-soluble organic liquid (exemplified by methyl isobutyl ketone) extractant was developed. This process has been described in Graham, World Petroleum Congress, proceedings, 7th, 1967 (Pub 1968), 5, 29-40 (Great Britain) and in U.S. Pat. Nos. 2,856,432; 2,856,433; 3,190,923; and 3,190,924. This process can be applied to the manufacture of p-diisopropylbenzene dihydroperoxide and insures that this material will enter the rearrangement reactor free of any benzene in the reaction solvent. It has the added benefit of a generally purer feed to the rearrangement reaction.
An application of the above methyl isobutyl ketone (MIBK) process to the manufacture of hydroquinone is described in Ewers, Voges, and Maleck, Erdoel Kohle Erdgas Petrochem, Br. Chem., 28(1) 34+(1975, West Germany). In this process (hereinafter referred to by the name of the owner, Veba-Chemie A.G.) the MIBK extract containing p-diisopropylbenzene dihydroperoxide is distillatively dried and then subjected to the Hock-splitting reaction. Reactor effluent is subjected to neutralization, distillation to remove the acetone formed in the reaction and part of the MIBK reaction solvent, extraction of the distillation bottoms with water, extraction of the hydroquinone-bearing aqueous extract with MIBK to remove impurities, concentration of the purified aqueous extract, and crystallization of the hydroquinone. Parts of this process are described in German Offenlegungsschrift No. 2446992, Apr. 15, 1976.
A process for purifying hydroquinone by successive aqueous extractions of the tars is described in U.S. Pat. No. 3,900,523. In that process it is a steam distillation following the rearrangement reaction which removes the reaction solvent and forms a crude hydroquinone aqueous solution consisting essentially of water, hydroquinone, solvent and tar substances. The concentration of hydroquinone in this aqueous solution must be about 23 weight percent or more at a temperature of about 60.degree. C.
The further purification of hydroquinone by recrystallization from acetone is described in Japanese Pat. Nos. 51039636 and 4872140 and German Offen. No. 2,541,489. In the German and the last mentioned Japanese documents the recrystallization is mentioned in connection with the MIBK process.
It is desirable to minimize or eliminate water from the hydroquinone purification because hydroquinone, in the presence of water (especially when heated) is subject to degradation to color bodies and polymeric material. Most of the aforementioned prior art patents are all aqueous processes. It has been found that advantages in product purity can be obtained if hydroquinone (formed in an MIBK-type solvent system by the Hock-splitting reaction) is maintained in a nonaqueous solvent system. A parallel process (involving a series of distillations and extractions) can be utilized to remove tars and separate and prepare hydroquinone carried with the tar-containing stream for recycle back to the main process equipment train. This combines the advantages of the MIBK process and extraction technology to keep water out of the main process stream.